For a gooseneck microphone used for conference and a tie clip microphone used by being attached to clothes among condenser microphones, a microphone unit and an output module section having an audio output circuit are separated from each other, and are connected to each other via a dedicated microphone cord. FIG. 3 is a sectional view showing a configuration on the microphone unit side of these two elements.
The microphone unit includes a microphone capsule 10 and a support enclosure 20 as a basic configuration. Although the microphone capsule 10 is exchangeably supported on the support enclosure 20 in this example, in some microphone unit, the microphone capsule 10 and the support enclosure 20 are integrated with each other.
The microphone capsule 10 has a cylindrical housing 11 made of, for example, a brass material. In the housing 11, a diaphragm 12 stretchedly provided on a support ring 13 and a backplate 14 supported on an insulating seat 15 are contained in a state of being arranged so as to face to each other via an electrical insulating spacer (not shown).
The back surface side of the housing 11 is closed by a back cover 16, and a contact pin 17, which is connected to the backplate 14 via a not illustrated wiring, protrudes from the back cover 16. Also, on the back surface side of the housing 11, a connection screw 18 for connecting the housing 11 to the support enclosure 20 is fixed so as to provide electrical continuity with the housing 11.
The support enclosure 20 is formed into a cylindrical shape by, for example, a brass material, and on one end side (upper end side in FIG. 3) thereof, a circuit board 21 is arranged so as to close the interior of the enclosure. On the circuit board 21, a field effect transistor (FET) 22 serving as an impedance converter is mounted. Although not shown in the figure, the circuit board 21 is formed with lead wires for electrodes of gate, drain, and source of the FET 22. The circuit board 21 is a double-sided circuit board, and at least the gate lead wire is pulled out to the top surface side of the circuit board 21 via a wire in a through hole.
On the circuit board 21, a contact terminal 23 consisting of a plate spring that is in contact with the contact pin 17 is mounted in a state of being held on a spacer 24 consisting of a rubber elastic element. The lower end of the contact terminal 23 is in contact with the gate lead wire of the FET 22. Therefore, when the microphone capsule 10 is connected to the support enclosure 20, the backplate 14 is connected to the gate of the FET 22 via the contact pin 17 and the contact terminal 23.
On the other end side (lower end side in FIG. 3) of the support enclosure 20, a cord introduction hole 25 having a cord bush 26 is provided, and a microphone cord 30 extending from this cord introduction hole 25 toward the output module section (not shown) side is pulled into the support enclosure 20. In the case of the gooseneck microphone, the microphone cord 30 is allowed to pass through a support pipe including a flexible pipe.
As the microphone cord 30, a two-core shield covering line is used which includes power line for supplying power to the microphone capsule 10, a signal line for sending an audio signal generated from the FET 22 to the output module section, not shown, and a shield covering line for electrostatically shielding and grounding the power line and signal line.
In a portion in which the microphone cord 30 is pulled into the support enclosure 20, a knot 30a for preventing the cord 30 from coming off is made, and for example, the power line is soldered to the gate lead wire of the FET 22, the signal line to the drain lead wire thereof, and the shield covering line to the source lead wire thereof. The source lead wire is laid out at the peripheral edge of the circuit board 21, and is brought into contact with the support enclosure 20.
Electromagnetic waves applied to the microphone cord 30 (a noise source for the microphone) are more or less shielded by the shield covering line. However, since the shield covering line of the microphon cord 30 is pulled into the support enclosure 20 together with the power line and signal line, the electromagnetic waves applied to the microphone cord 30 intrude into the support enclosure 20, and are detected by the FET 22, so that noise is sometimes generated.
In recent years, cellular phones have come into wide use rapidly. In the case where a cellular phone is used in the immediate vicinity of a microphone, the microphone receives considerably strong electromagnetic waves (for example, in the range of several centimeters to several tens centimeters, field intensity reaching several ten thousands times of intensity of field generated in the city by commercial electric waves), so that measures against cellular phones are a pressing need in the field of microphone.
As one method for answering the need, a technique in which, for example, in a gun microphone in which the microphone unit is housed in a housing cylinder consisting of a conductor, the microphone unit is connected (grounded) to the housing cylinder consisting of a conductor at the shortest distance has been proposed in Japanese Patent Application Publication No. 2001-103591.
However, although being effective for the gun microphone or the like, this method cannot be applied to a microphone in which, as in the conventional example, the microphone capsule and the output module section are separated from each other and are connected to each other via the microphone cord.